Satellite communications system employing frequency reuse

ABSTRACT

A satellite communications system employs separate subsystems for broadcast and point-to-point two-way communications using the same assigned frequency band and employs an antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high-gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones to permit multiple reuse of the entire band of assigned frequencies. A single pool of solid-state transmitters allows rain-disadvantaged users to be assigned higher than normal power at minimum cost and geographically disperses the transmitter intermodulation products. In an alternate embodiment, the satellite (200) employs direct radiating array antennas (202, 204) for reception and transmission. The system (200 ) utilizes hybrid-coupled dual amplifiers (251) to reduce amplifier production costs. In another embodiment, both point-to-point and broadcast services are available on a single polarization by allocating one-half of the frequency spectrum to each service and by using separate direct radiating arrays from horizontal and vertical polarization for both reception (235, 236) and transmission (237, 238). The frequency spectrum is reused in each of the contiguous receive zones (220, 222, 224, 226) and the transmit zones (228, 230, 232, 234) because sufficient spatial isolation is achieved by subdividing the receive zones in two halves (220a, 220b, 222a, 222b, 224a, 224b, 226a, 226b), and by using one-half of the frequency spectrum in each subdivided zone.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 896,983, filed Aug. 14, 1986, now Patent 4,819,227.

TECHNICAL FIELD

The present invention broadly relates to satellite communication systemsespecially of the type employing a satellite placed in geosynchronousorbit above the earth so as to form a communication link between manysmall aperture terminals on the earth. More particularly, the inventioninvolves a communication satellite having hybrid communicationcapability accommodating both two-way and broadcast communicationsystems. Two-way communications between small aperture earth terminalsis achieved through multi-fold reuse of a fixed frequency spectrum incontiguous zones of an area on the earth.

BACKGROUND ART

In domestic communication satellite systems, which interconnect largenumbers of very small aperture earth terminals, the most importantparameters affecting the system capacity are the Effective IsotropicRadiated Power (EIRP) and the available bandwidth. EIRP refers to ameasure of the satellite's transmitter power which takes intoconsideration the gain of the antenna. EIRP is the power of atransmitter and isotropic antenna that would achieve the same result asthe transmitter and antenna which is actually employed.

In the past, high antenna gain and multiple frequency reuse has beenachieved by employing a plurality of up and down link beams covering theregions of a country or other areas of the earth to be served. Bothfrequency division and time division systems have been used or proposedto interconnect large numbers of signals from many geographicallyseparated earth stations. Time division systems permit the satellitetransmitters to operate efficiently. This is because only one timedivision signal at a time is amplified in a transmitter, so it may beoperated at or close to signal channel saturation, the most efficientoperating point. However, time division systems require high powerground transmitters and expensive signal processing and are thereforeincompatible with low cost earth stations. Frequency division systemsare better suited to low cost earth station, but have lower satellitetransmitter efficiency because each transmitter handles multiplecarriers. Since multiple carrier amplifiers generate undesirableintermodulation products that increase in power as the transmitterefficiency is increased, the optimum compromise between transmitterefficiency and intermodulation product generation results in arelatively low transmitter efficiency.

In Ku band, the satellite communication band most suitable for two-wayservice between very small terminals, the attenuation of the signals byrain is an important consideration in the design of the system. In theprevious systems, this attenuation is overcome on the downlink by usinghigher satellite transmitter power per channel than would be necessaryfor clear weather service, typically four times as much. Thisaccommodation of rain attenuation therefore results in more expensivesatellites having fewer available channels.

The available bandwidth of a satellite system is determined by thenumber of times the allocated frequency spectrum can be reused.Polarization and spatial isolation of beams have been employed to permitreuse of the frequency spectrum. As the number of isolated beams isincreased, however, the problem of interconnecting all the users becomesvery complicated and is one of the factors that limit the number ofreuses of the frequency spectrum.

The present invention is directed toward overcoming each of thedeficiencies mentioned above.

SUMMARY OF THE INVENTION

The present invention provides a satellite communication system forinterconnecting large numbers of very small aperture earth terminals andmobile satellite service users which maximizes satellite EIRP as well asthe available bandwidth. The system employs highly directional,contiguous beams on the downlink or transmit signal which substantiallyincreases the EIRP and allows multiple reuse of the assigned frequencyspectrum. As a result, the number of communications that can be providedfor point-to-point service is maximized. High multi-carrier transmitterefficiency is achieved as a result of the dispersion of intermodulationproducts and the deleterious effects of rain on the downlink channelsare easily overcome by the use of pooled transmitter power. Theinterconnection of the many users is achieved by a combination of afilter interconnection matrix and a highly directional addressabledownlink beam.

According to the present invention, a system is provided forinterconnecting any of a plurality of earth terminals or mobileterminals within an area on the earth for two-way communication using acommunications satellite. A plurality of uplink beams are formed whichrespectively emanate from contiguous zones covering the area to beserviced by the satellite. The uplink beams carry a plurality ofchannels over a first preset range of uplink frequencies. Each uplinkzone uses the same preset range of frequencies. The uplink frequenciesare therefore reused by each zone, thereby effectively multiplying thenumber of communications channels that can be handled by the satellite.A plurality of downlink beams destined for the downlink zones also carrya plurality of channels over a second preset range of frequencies. Thebeams for each of the downlink zones also use the same second presetrange of frequencies to provide multiple reuse of these frequencies. Thesatellite employs a filter interconnection matrix for interconnectingthe channels in the different zones.

The preset range of frequencies are separated into first and secondsets. The preset range of frequencies is spatially distributed over eachzone such that contiguous regions of adjacent zones are not serviced bythe same frequency set. Since contiguous regions of adjacent zonescommunicate over different frequency sets, there is sufficient spatialisolation between contiguous zones to permit frequency reuse. The beamsoperating over the first frequency set are simultaneously transmitted toall of the zones and then the beams operating over the second set aresimultaneously transmitted to all the zones. This is achieved by using atwo-position switch which insures that beams operating over the firstand second sets are alternately transmitted.

A fan beam narrow in one direction, east-west for example, and broad inthe orthogonal direction, is generated by a beam-forming network used inconjunction with the transmit array antenna. The transmit array may be aconfocal arrangement or a direct radiating array. The east-westdirection of the beam within the covered area is determined by thedownlink frequency, which is related to the uplink frequency by aconstant difference. The uplink frequency therefore determines thedownlink frequency, which by action of the beam-forming network and thetransmit array determine the direction and hence, the destination of thedownlink beam. Such an arrangement is referred to as a frequencyaddressable beam. The side lobes of the beam are designed to be lowenough to permit reuse of the frequency spectrum in the adjacent zones.

The transmitters, preferably equal power dual solid state amplifiers,are embedded in the transmit array antenna, one amplifier beingassociated with two staves of the array. All of the amplifiers operateat the same power level despite the unequal power inputs and outputs.Since all of the downlink power is provided by this single pool oftransmitters, it is easy to provide relatively high power to thoserelatively few signals directed to rain affected areas with only a verysmall reduction in power available to the much larger number ofunimpaired signals.

Because the transmit beam directions are related to the frequencies oftheir signals, and the frequencies of the intermodulation productsgenerated in the power amplifiers differ from those of the signals whichcause them, the intermodulation products go down in different directionsthan the signals. This process results in the spatial dispersion of theintermodulation products. This dispersion is enhanced by the use ofmultiple downlink zones. This results in a lower intermodulation productdensity at all ground terminals in the frequency bands to which they aretuned. This reduced sensitivity to intermodulation products permits thepower amplifiers in the satellite to be operated more efficiently.

It is therefore a primary object of the invention to provide acommunication satellite for interconnecting large numbers of very smallaperture antenna terminals and mobile satellite service users using highsatellite transmit antenna gain and allowing multiple reuse of theassigned frequency spectrum, to substantially increase the number ofcommunication channels that can be provided for point-to-pointcommunication service.

Another object of the invention is to provide the downlink power from asingle pool of transmitters, so that signals being attenuated by raincan be easily allocated more satellite transmit power.

Another object of the invention is to provide a communication satellitewhich disperses intermodulation products in order to increasetransmitter efficiency.

A further object of the invention is to provide a communicationsatellite as described above which provides both broadcast, andpoint-to-point communications service.

A further object of the invention is to provide a communicationsatellite which uses direct radiating array antennas in a system whichreuses the assigned frequency spectrum.

Another object of the invention is to provide point-to-point andbroadcast services on a single polarization.

These, and further objects and advantages of the invention will be madeclear or will become apparent during the course of the followingdescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a perspective view of a communications satellite, showing theantenna subsystems;

FIG. 2 is a top plan view of the antenna subsystems shown in FIG. 1;

FIG. 3 is a sectional view taken along the line 3--3 in FIG. 2;

FIG. 4 is a sectional view taken along the line 4--4 in FIG. 2;

FIG. 5 is a view of the United States and depicts multiple, contiguousreceive zones covered by the satellite of the present invention, theprimary areas of coverage being indicated in cross-hatching and theareas of contention being indicated by a dimpled pattern;

FIG. 6 is a block diagram of the communication electronics for thecommunications satellite;

FIG. 7 is a schematic diagram of a coupling network which interconnectsthe point-to-point receive feed horns with the inputs to thecommunications electronics shown in FIG. 6;

FIG. 8 is a reference table of the interconnect channels employed toconnect the receive and transmit zones for the point-to-point system;

FIG. 9 is a diagrammatic view of the United States depicting multiplecontiguous transmit zones covered by the satellite and the geographicdistribution of the interconnected channels for each zone, across theUnited States;

FIG. 9A is a graph showing the variation in gain of the transmit antennabeam for each zone in the point-to-point system in relation to thedistance from the center of the beam in the east-west direction;

FIG. 9B is a graph similar to FIG. 9A but showing the variation in gainin the north-south direction;

FIG. 10 is a detailed schematic diagram of the filter interconnectionmatrix employed in the point-to-point system;

FIG. 11 is a detailed, plan view of the beam-forming network employed inthe point-to-point system;

FIG. 12 is an enlarged, fragmentary view of a portion of thebeam-forming network shown in FIG. 11;

FIG. 13 is a front elevational view of the transmit array for thepoint-to-point system, the horizontal slots in each transmit element notbeing shown for sake of simplicity;

FIG. 14 is a side view of the transmit element of the array shown inFIG. 13 and depicting a corporate feed network for the element;

FIG. 15 is a front, perspective view of one of the transmit elementsemployed in the transmit array of FIG. 13;

FIG. 16 is a front view of the receive feed horns for the point-to-pointsystem;

FIG. 17 is a diagrammatic view showing the relationship between atransmitted wave and a portion of the transmit feed array for thepoint-to-point system;

FIG. 18 is a perspective view of a deployed mobile satellite which formsan alternate embodiment of the present invention;

FIG. 19 is similar to FIG. 18 but depicts the mobile satellite in itsstowed position;

FIG. 20 is an elevational view of a directly-radiating array antennaforming part of the satellite of FIG. 18;

FIG. 21 is a view similar to FIG. 5 but depicting zones covered by thesatellite of FIG. 18;

FIG. 22 is an illustration similar to FIG. 9A, but showing the antennapattern contours for three zones;

FIG. 23 is a diagrammatic view showing a beam-forming network designedfor a system which reuses the frequency spectrum three times;

FIG. 24 is similar to FIG. 6, but illustrates a block diagram for thealternate embodiment;

FIG. 25 is a diagrammatic view of an equal power dual amplifier and theresulting power distribution;

FIG. 26 is a perspective view of another deployed satellite which formsa further embodiment of the present invention;

FIG. 27 is a view similar to FIG. 5 and FIG. 21, but depicting alternatereceive zones;

FIG. 28 is an alternate illustration, similar to FIG. 9A and FIG. 22, ofthe receive beam patterns;

FIG. 29 is similar to FIG. 9 illustrating alternate transmit zones;

FIG. 30 is an illustration of the transmit beam pattern corresponding tothe transmit zones in FIG. 29;

FIG. 31 is a reference table similar to that shown in FIG. 8, but for afilter interconnection matrix for the mobile satellite system depictedin FIG. 26; and

FIG. 32 is a block diagram of the repeater employed in the satelliteshown in FIG. 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1-4, a communications satellite 10 is depictedwhich is placed in geosynchronous orbit above the earth's surface. Thesatellite's antenna system, which will be described in more detailbelow, will typically be mounted on an earth-oriented platform so thatthe antenna system maintains a constant orientation with respect to theearth.

The satellite 10 is of a hybrid communications-type satellite whichprovides two different types of communication services in a particularfrequency band, for example, the fixed satellite service Ku band. Onetype of communication service, referred to hereinafter as point-to-pointservice, provides two-way communications between very small apertureantenna terminals of relatively narrow band voice and data signals.Through the use of frequency division multiple access (FDMA) and reuseof the assigned frequency spectrum, tens of thousands of suchcommunication channels are accommodated simultaneously on a singlelinear polarization. The other type of communication service provided bythe satellite 10 is a broadcast service, and it is carried on the otherlinear polarization. The broadcast service is primarily used for one-waydistribution of video and data throughout the geographic territoryserved by the satellite 10. As such, the transmit antenna beam coversthe entire geographic territory. For illustrative purposes throughoutthis description, it will be assumed that the geographic area to beserviced by both the point-to-point and broadcast services will be theUnited States. Accordingly, the broadcast service will be referred tohereinafter as CONUS (Continental United States).

The antenna system of the satellite 10 includes a conventional omniantenna 13 and two antenna subsystems for respectively servicing thepoint-to-point and CONUS systems. The point-to-point antenna subsystemprovides a two-way communication link to interconnect earth stations fortwo- way communications. The CONUS antenna system functions as atransponder to broadcast, over a wide pattern covering the entire UnitedStates, signals received by one or more particular locations on earth.The point-to-point transmit signal and the CONUS receive signal arevertically polarized. The CONUS transmit and point-to-point receivesignals are horizontally polarized. The antenna system includes a largereflector assembly 12 comprising two reflectors 12a, 12b. The tworeflectors 12a, 12b are rotated relative to each other about a commonaxis and intersect at their midpoints. The reflector 12a is horizontallypolarized and operates with horizontally polarized signals, while thereflector 12b is vertically polarized and therefore operates withvertically polarized signals. Consequently, each of the reflectors 12a,12b reflects signals which the other reflector 12a, 12b transmits.

A frequency selective screen 18 is provided which includes two halves orsections 18a, 18b and is mounted on a support 30 such that the screenhalves 18a, 18b are disposed on opposite sides of a centerline passingdiametrically through the satellite 10, as best seen in FIG. 2. Thefrequency selective screen 18 functions as a diplexer for separatingdifferent bands of frequencies and may comprise an array of discrete,electrically conductive elements formed of any suitable material, suchas copper. Any of various types of known frequency selective screens maybe employed in this antenna system. However, one suitable frequencyselective screen, exhibiting sharp transition characteristics andcapable of separating two frequency bands which are relatively close toeach other, is described in U.S. patent application Ser. No. 896,534,filed Aug. 14, 1986, and assigned to Hughes Aircraft Company. Thefrequency selective screen 18 effectively separates the transmitted andreceived signals for both the CONUS and point-to-point subsystems. Itmay be appreciated that the two halves 18a, 18b of the screen 18 arerespectively adapted to separate individual signals which arehorizontally and vertically polarized.

The CONUS subsystem, which serves the entire country with a single beamhas, in this example, eight conventional transponders each having a highpower traveling wave tube amplifier as its transmitter 82 (see FIG. 6).The CONUS receive antenna uses vertical polarization, sharing thevertically polarized reflector 12b with the point-to-point transmissionsystem. CONUS receive signals pass through the frequency selectivescreen half 18b and are focused on the receive feed horns 14 located atthe focal plane 28 of reflector 12b. The antenna pattern so formed isshaped to cover CONUS. The CONUS transmit antenna employs horizontalpolarization, and shares reflector 12a with the point-to-point receivesystem. Signals radiating from the transmit feeds 24 are reflected bythe horizontally polarized frequency selective screen 18a to reflector12a whose secondary pattern is shaped to cover CONUS.

The point-to-point subsystem broadly includes a transmit array 20, asubreflector 22, and receive feed horns 16. The transmit array 20, whichwill be described later in more detail, is mounted on the support 30,immediately beneath the screen 18. The subreflector 22 is mountedforward of the transmit array 20 and slightly below the screen 18. Thesignal emanating from the transmit array 20 is reflected by thesubreflector 22 onto one half 18b of the screen 18. The subreflector 22in conjunction with the main reflector 12 functions to effectivelymagnify and enlarge the pattern of the signal emanating from thetransmit array 20. The signal reflected from the subreflector 22 is, inturn, reflected by one half 18b of the screen 18 onto the largereflector 12b, which in turn reflects the point-to-point signal to theearth. Through this arrangement, the performance of a large aperturephase array is achieved. The receive feed horns 16 are positioned in thefocal plane 26 of the reflector 12a. It consists of four main horns 50,54, 58, 62 and three auxiliary horns 52, 56, 60 as shown in FIG. 16.

Referring now also to FIGS. 13-15, the transmit array 20 comprises aplurality, for example forty, transmit waveguide elements 106 disposedin side-by-side relationship to form an array, as shown in FIG. 13. Eachof the transmit waveguide elements 106 includes a plurality, for exampletwenty-six, of horizontal, vertically spaced slots 108 therein whichresult in the generation of a vertically polarized signal. As shown inFIG. 14, the transmit array 20 is fed with a transmit signal by means ofa corporate feed network, generally indicated by the numeral 110 whichexcites the array element in four places 114. The purpose of thecorporate feed network 110 is to provide a broadband match to thetransmit waveguide element 106. Signals input to the waveguide opening112 excite the array slots 108 so that the slot excitation is designedto give a flat pattern in the north-south direction.

Attention is now directed to FIG. 5 which depicts a generallyrectangular beam coverage provided by the horizontally polarizedpoint-to-point receive system. In this particular example, the areaserviced by the point-to-point system is the continental United States.The point-to-point receive system comprises four beams R1, R2, R3, R4respectively emanating from the four uplink zones 32, 34, 36, 38 to thesatellite, wherein each of the beams R1-R4 consists of a plurality ofindividual uplink beams originating from individual sites in each zone32, 34, 36, 38 and carries an individual signal from that site. Theuplink beam signals from the individual sites are arranged into aplurality of channels for each zone. For example, zone 32 may include aplurality, e.g. sixteen 27 MHz channels with each of such channelscarrying hundreds of individual beam signals from corresponding uplinksites in zone 32.

The signal strength for each of the four beam pattern contours,respectively designated by numerals 32, 34, 36 and 38, are approximately3 dB down from peaks of their respective beams. The antenna beams havebeen designed to achieve sufficient isolation between them to makefeasible in the cross-hatched regions 39, 41, 43, 45 reuse of thefrequency spectrum four times. In the dotted regions 40, 42, and 44, theisolation is insufficient to distinguish between signals of the samefrequency originating in adjacent zones. Each signal originating inthese regions will generate two downlink signals, one intended and oneextraneous. The generation of extraneous signals in these areas will bediscussed later in more detail.

It may be readily appreciated from FIG. 5 that the four zones covered bybeams 32, 34, 36, 38 are unequal in width. The East Coast zone coveredby beam 32 extends approximately 1.2 degrees; the Central zone coveredby beam 34 extends approximately 1.2 degrees; the Midwest zone coveredby beam pattern 36 extends approximately 20 degrees, and; the West Coastzone covered by beam pattern 38 extends approximately 2.0 degrees. Thewidth of each of the four receive zones 32, 34, 36 and 38 is determinedby the number of terminals and thus the population density in thevarious regions of the country. Thus, beam pattern 32 is relativelynarrow to accommodate the relatively high population density in theEastern part of the United States while beam pattern 36 is relativelywide due to the relatively low population density in the Mountainstates. Since each zone utilizes the entire frequency spectrum, zonewidths are narrower in regions where the population density is high, toaccommodate the greater demand for channel usage.

As shown in FIG. 9, the point-to-point transmit system comprises fourbeams T1, T2, T3, T4 respectively covering the four transmit zones 31,33, 35, 37, wherein each of the beams T1-T4 consists of a plurality ofindividual downlink beams destined for the individual downlink sites ineach zone 31, 33, 35, 37 and carries an individual signal to that site.The downlink beam signals, destined to be received at the individualdownlink sites, are arranged into a plurality of channels for each zone.For example, zone 31 may include a plurality, e.g. sixteen 27 MHzchannels with each of such channels carrying hundreds of individual beamsignals to corresponding downlink sites in zone 32.

The use of multiple downlink zones and downlink zones of unequal widthsassist in causing the intermodulation products, generated by thelater-discussed solid state power amplifiers, to be geographicallydispersed in a manner which prevents most of these products from beingreceived at the ground terminals. The net effect is that the amplifiersmay be operated more efficiently because the system can tolerate moreintermodulation products. Although the widths of the transmit zones 31,33, 35, 37 are nearly the same as those of the receive zones R1, R2, R3,R4, small differences between the two sets have been found to optimizethe capacity of the system.

The half power beam width of the individual transmit beams 29 issubstantially narrower than that of the transmit zones 31, 33, 35, 37.This results in the desirable high gain, and avoids the zones ofcontention 40, 42, 44 characteristic of the receive zone arrangement.These individual beams 29 must be steered within the zones in order tomaximize the downlink EIRP in the directions of the individualdestination terminals. The transmit point-to-point frequency addressablenarrow beams 29 are generated by an array 20 whose apparent size ismagnified by two confocal parabolas comprising a main reflector 12b anda subreflector 22. The east-west direction of each beam 29 is determinedby the phase progression of its signal along the array 106 of transmitelements 20 (FIGS. 13 and 15). This phase progression is established bya later-discussed beam-forming network 98 and is a function of thesignal frequency. Each of the transmit array elements 20 is driven by alater-discussed solid state power amplifier. The power delivered to thearray elements 106 is not uniform but is instead tapered with the edgeelements being more than 10 dB down. Tapering of the beams 29 isachieved by adjusting the transmit gain according to the position of thetransmit array elements 20. The excitation pattern determines thecharacteristics of the transmit secondary pattern, shown in FIG. 9A.Referring to FIG. 9, the closest spacing between transmit zones 31, 33,35, 37 occurs between zones 31 and 33 and is approximately 1.2 degrees.This means that a signal addressed to zone 33 using a particularfrequency would interfere with a signal using the same frequency in zone31 with its side lobe 1.2 degrees from its beam center. However, theindividual transmit gains have been adjusted to provide low side lobelevels, thereby permitting frequency reuse in adjacent zones. Referringto FIG. 9A, it is seen that the side lobe level at this angle off beamcenter is more than 30 dB down, so that such interference will benegligibly small. The same frequency uses in zones 35 and 37 are furtherremoved in angle, hence the side lobe interference in those zones iseven smaller.

FIG. 9B is an illustration of the transmit beam pattern in thenorth-south direction. The twenty six slots 108 in each of the transmitwaveguide elements 106 are excited in a manner which creates a nearlyflat north-south pattern, extending over the covered range of plus andminus 1.4 degrees from the north-south boresight direction.

Both the point-to-point and CONUS systems may utilize the same uplinkand downlink frequency bands, with the point-to-point system usinghorizontal polarization for its uplink polarization, and the CONUSsystem using vertical polarization, as previously mentioned. Forexample, both services may, simultaneously, utilize the entire 500 MHzuplink frequency band between 14 and 14.5 GHz, as well as the entire 500MHz downlink frequency band between 11.7 and 12.2 GHz. Each of thereceive zones 32, 34, 36, 38 and transmit zones 31, 33, 35, 37,employing the point-to-point service utilizes the entire frequencyspectrum (i.e. 500 MHz). Furthermore, this total frequency spectrum isdivided into a plurality of channels, for example, sixteen channels eachhaving a usable bandwidth of 27 MHz and a spacing of 30 MHz. In turn,each of the sixteen channels may accommodate approximately 800subchannels. Hence, within each zone, approximately 12,500 (16channels×800 subchannels) 32 kilobit per second channels may beaccommodated, at any given moment. As will be discussed below, thecommunication architecture of the point-to-point system allows anyterminal to communicate directly with any other terminal. Thus, within asingle polarization, a total of 50,000 subchannels may be accommodatednationwide.

Referring now particularly to FIGS. 1, 2, 6, 7 and 16, thepoint-to-point receive feed array 16 employs seven receive horns 50-62.Horns 50, 54, 58 and 62 respectively receive signals from zones 32, 34,36 and 38. Horns 52, 56 and 60 receive signals from the zones ofcontention 40, 42 and 44. Using a series of hybrid couplers or powerdividers C₁ -C₉, the signals received by horns 50-62 are combined intofour outputs 64-70. For example, a signal originating from the area ofcontention 44 and received by horn 60 is divided by coupler C₂ andportions of the divided signal are respectively delivered to couplers C₁and coupler C₄ whereby the split signal is combined with the incomingsignals received by horns 58, 62 respectively. Similarly, signalsoriginating from the area of contention 42 and received by horn 56 aresplit by coupler C₅. A portion of the split signal is combined bycoupler C₃, with the signal output of coupler C₄, while the remainingportion of the split signal is combined, by coupler C.sub. 7, with thesignal received by horn 54.

Attention is now particularly directed to FIG. 6 which depicts, in blockdiagram form, the electronics for receiving and transmitting signals forboth the CONUS and point-to-point systems. The point-to-point receivesignals 64-70 (see also FIG. 7) are derived from the point-to-pointreceive feed network in FIG. 7, whereas the CONUS receive signal 72derives from the CONUS receive feed horns 14, (FIGS. 1 and 3). Both thepoint-to-point and CONUS receive signal are input to a switching network76 which selectively connects input lines 64-72 with five correspondingreceivers, eight of which receivers are generally indicated at 74. Thereceivers 74 are of conventional design, three of which are provided forredundancy and are not normally used unless a malfunction in one of thereceivers is experienced. In the event of a malfunction, switchingnetwork 76 reconnects the appropriate incoming line 64-72 with a back-upreceiver 74. Receivers 74 function to drive the filters in a filterinterconnection matrix 90. The outputs of the receivers 74, which areconnected with lines 64-70, are coupled by a second switching network 78through four receive lines R1-R4 to a filter interconnection matrix 90.As will be discussed later below, the filter interconnection matrix(FIM) provides interconnections between the receive zones 32, 34, 36,38, and the transmit zones 31, 33, 35, 37. Operating in theabove-mentioned 500 MHz assigned frequency spectrum, separated intosixteen 27 MHz channels, four sets of sixteen filters are employed. Eachset of the sixteen filters utilizes the entire 500 MHz frequencyspectrum and each filter has a 27 MHz bandwidth. As will be discussedlater, the filter outputs T1-T4 are arranged in four groups, each groupdestined for one of the four transmit zones 31, 33, 35, 37.

The transmit signals T1-T4 are respectively connected, via switchingnetwork 94, to four of six driving amplifiers 92, two of such amplifiers92 being provided for back-up in the event of failure. In the event ofthe failure of one of the amplifiers 92, one of the back-up amplifiers92 will be reconnected to the corresponding transmit signal T1-T4 by theswitching network 94. A similar switching network 96 couples theamplified output of the amplifiers 92 to a beam-forming network 98. Aswill be discussed later in more detail, the beam-forming network 98consists of a plurality of transmission delay lines connected at equalintervals along the four delay lines. These intervals and the width ofthe delay lines are chosen to provide the desired centerband beam squintand the beam scan rate with frequency for the corresponding transmitzones 31, 33, 35, 37 to be serviced. The transmit signals, coupled fromthe four delay lines, are summed in the beam-forming network 98 as shownin FIGS. 11 and 12, to provide inputs to solid state power amplifiers100, which may be embedded in the point-to-point system's transmit array20. In the illustrated embodiment discussed below, forty solid statepower amplifiers (SSPAs) 100 are provided. Each of the SSPAs 100amplifies a corresponding one of the forty signals formed by thebeam-forming network 98. The SSPAs 100 posses different power capacitiesto provide the tapered array excitation previously mentioned. The outputof the SSPA 100 is connected to the input 112 (FIG. 14) at one of theelements of the transmit array 20.

The receive signal for CONUS on line 72 is connected to an appropriatereceiver 74 by switching networks 76, 78. The output of the receiverconnected with the CONUS signal is delivered to an input multiplexer 80which provides for eight channels, as mentioned above. The purpose ofthe input multiplexers 80 is to divide the one low level CONUS signalinto subsignals so that the subsignals can be amplified on an individualbasis. The CONUS receive signals are highly amplified so that the CONUStransmit signal may be distributed to very small earth terminals. Theoutputs of the input multiplexer 80 are connected through a switchingnetwork 84 to eight of twelve high power traveling wave tube amplifiers(TWTAs) 82, four of which TWTAs 82 are employed for back-up in the eventof failure. The outputs of the eight TWTAs 82 are connected throughanother switching network 86 to an output multiplexer 88 whichrecombines the eight amplified signals to form one CONUS transmitsignal. The output of the multiplexer 88 is delivered via waveguide tothe transmit horns of the CONUS transmitter 24 (FIGS. 2 and 3).

Attention is now directed to FIG. 10 which depicts the details of theFIM 90 (FIG. 6). As previously discussed, the FIM 90 effectivelyinterconnects any terminal in any of the receive zones 32, 34, 36, 38(FIG. 5) with any terminal in any of the transmit zones 31, 33, 35, 37.The FIM 90 includes four waveguide inputs 120, 122, 124 and 126 forrespectively receiving the receive signals R1, R2, R3 and R4. Aspreviously mentioned, receive signals R1-R4, which originate from acorresponding receive zone 32, 34, 36, 38 (FIG. 5), each contain theentire assigned frequency spectrum, (e.g. 500 MHz), and are separatedinto a plurality of channels (e.g. sixteen 27 MHz channels). Thechannels are further separated into a plurality of subchannels, whereeach of the subchannels carries a signal from a corresponding uplinksite. The FIM 90 includes 64 filters, one of which is indicated by thenumeral 102. Each of the filters 102 has a passband corresponding to oneof the channels (e.g. 1403-1430 MHz). The filters 102 are arranged infour groups, one for each receive zone 32, 34, 36, 38, with each groupincluding two banks or subgroups of eight filters per subgroup. Onesubgroup of filters 102 contains those filters for the even-numberedchannels and the other subgroup in each group contains eight filters forthe odd-numbered channels. Thus, for example, the filter group forreceive signal R1 comprises subgroup 104 of filters 102 for oddchannels, and subgroup 106 of filters 102 for even channels. Thefollowing table relates the receive signals and zones to their filtersubgroups:

    ______________________________________                                                        Filter Subgroups                                              Receive Zone                                                                           Receive Signal                                                                             Odd Channels                                                                              Even Channels                               ______________________________________                                        32       R1           104         106                                         34       R2           108         110                                         36       R3           112         114                                         38       R4           116         118                                         ______________________________________                                    

The filters are grouped in a unique manner such that when the receivesignals R1-R4 are filtered, the filtered outputs are combined to formthe transmit signals. The transmit signal T1-T4 also utilize the entireassigned frequency spectrum, (e.g. 500 MHz). In the illustratedembodiment, each of the transmit signals T1-T4 possesses sixteen 27 MHzwide channels, and comprises four channels from each of the four receivezones 32-38 (FIG. 5).

The incoming receive signals R1-R4 are divided into the correspondingsubgroups by respectively associated hybrid couplers 128-134 whicheffectively divert 50% of the signal power to each subgroup. Hence, forexample, one-half of the R1 signal input as waveguide 120 is diverted totransmission line 136 which services the subgroup 104 of filters 102,and the remaining half of the R1 signal is diverted to transmission line138 which services subgroup 106 of filters 102. In a similar manner,each of the subgroups 104-118 of filters 102 is served by acorresponding distribution line, similar to lines 136 and 138.

The construction of subgroup 104 will now be described in more detail,it being understood that the remaining subgroups 106-118 are identicalin architecture to subgroup 104. At intervals along the transmissionline 136, there are eight ferrite circulators 140, one associated witheach of the odd-numbered channel filters 102. The function of thecirculators 140 is to connect the transmission line 136 to each of theodd channel filters 102 in a lossless manner. Thus, for example, the R1signal enters the first circulator 140a and circulates itcounterclockwise whereby the 27 MHz band of signals corresponding tochannel 1 passes through it to circulator 142. All other frequencies arereflected. These reflected signals propagate via the circulator towardthe next filter where the process is repeated. Through this process, theR1 receive signal is filtered into sixteen channels by the sixteenfilters 104-108 corresponding to the R1 signals. Hence, the R1 signalwith frequencies in the range of channel 1 will pass through the firstferrite circulator 140a and it will be filtered by filter 1 of group104.

The outputs from a filter subgroup 104-118 are selectively coupled by asecond set of ferrite circulators 142 which sums, in a criss-crosspattern, the outputs from an adjacent group of filters 102. For example,the outputs of channel filters 1, 5, 9, and 13 of group 104 are summedwith the outputs of channel filters 3, 7, 11 and 15 of filter group 112.This sum appears at the output terminal for T1 144. Referring to FIG. 8,these signals correspond to the connections between receive zones R1 andR3 and to transmit zone T1.

Attention is now directed to FIGS. 8 and 9 which depict how the transmitand receive signals are interconnected by the FIM 90 to allow two-waycommunication between any terminals. Specifically, FIG. 8 provides atable showing how the receive and transmit zones are connected togetherby the interconnect channels while FIG. 9 depicts how these interconnectchannels are distributed geographically across the transmit zones 31,33, 35, 37. In FIG. 8, the receive signals R1-R4 are read across by rowsof interconnect channels and the transmit signals T1-T4 are read bycolumns of interconnect channels. In can be readily appreciated fromFIG. 8 that each of the transmit signals T1-T4 is made up of sixteenchannels arranged in four groups respectively, where each group isassociated with one of the receive signals R1-R4. The satellitecommunications system of the present invention is intended to be used inconjunction with a ground station referred to as a satellite networkcontrol center which coordinates communications between the groundterminals via packet switched signals. The network control centerassigns an uplink user with an uplink frequency based on the location ofthe desired downlink, assigning the available frequency whose downlinklongitude is closest to that of the destination. The frequencyaddressable downlink transmit beams 29 are thus addressable by thefrequencies of the uplink signals. This strategy maximizes the gain ofthe downlink signal.

As shown in FIG. 9, the continental United States is divided into fourprimary zones 31, 33, 35, 37. Zone 31 may be referred to as the EastCoast zone, zone 33 is the Central zone, zone 35 is the Mountain zone,and zone 37 is the West Coast zone. As previously mentioned, each of thezones 31, 33, 35, 37 utilizes the entire assigned frequency spectrum(e.g. 500 MHz). Thus, in the case of a 500 MHz assigned frequency band,there exists sixteen 27 MHz channels plus guard bands in each of thezones 31, 33, 35, 37.

The numbers 1-16 repeated four times above the beams 29 in FIG. 9indicate the longitude of the beams corresponding to the centerfrequencies of the channels so numbered. Because of the frequencysensitivity of the beams, the longitude span between the lowest andhighest frequency narrow band signal in a channel is approximately onechannel width. Each beam is 0.6 degrees wide between its half powerpoint, about half the zone width in the East Coast and Central zones andnearly one-third the zone width in the Mountain and West Coast zones.The antenna beams 29 overlap each other to ensure a high signal density;the more that the beams overlap, the greater channel capacity in a givenarea. Hence, in the East Coast zone 31, there is a greater overlap thanin the Mountain zone 35 because the signal traffic in the East Coastzone 31 is considerably greater than that in the Mountain zone 35.

The interconnect scheme described above will now be explained by way ofa typical communication between terminals in different zones. In thisexample, it will be assumed that a caller in Detroit, Mich. wishes toplace a call to a terminal in Los Angeles, Calif. Thus, Detroit, Mich.,which is located in the Central zone 24, is the uplink site, and LosAngeles, Calif., which is located in the West Coast zone 37, is thedownlink destination. As shown in FIG. 9, each geographic location inthe continental United States can be associated with a specific channelin a specific zone. Thus, Los Angeles is positioned between channels 14and 15 in transmit zone 37.

Referring now concurrently to FIGS. 5, 8 and 9 particularly, receive andtransmit zones R1 and T1 lie within the East Coast zone 32 and 31, R2and T2 lie within the Central zone 34 and 33, R3 and T3 lie within theMountain zone 36 and 35, and R4 and T4 lie within the West Coast zone 38and 37. Since Detroit lies in the Central or R2 zone 34, it can be seenthat the only channels over which signals can be transmitted to the WestCoast of T4 zone 37 are channels 1, 5, 9 and 13. This is determined inthe table of FIG. 8 by the intersection of row R2 and column T4.Therefore, from Detroit, the uplink user would uplink on either channel1, 5, 9 or 13, whichever of these channels is closest to the downlinkdestination. Since Los Angeles is located between channels 14 and 15,the network control center would uplink the signal on channel 13 becausechannel 13 is the closest to channel 14. The downlink beam width isbroad enough to provide high gain at Los Angeles.

Conversely, if the uplink site in Los Angeles and the downlinkdestination is in Detroit, the intersection of row R4 and column T2 inFIG. 8 must be consulted. This intersection reveals that the signal canbe transmitted on channels 1, 5, 9 or 13 depending upon which channel isclosest to the downlink destination. The network control center woulduplink the signal from Los Angeles on channel 9 since channel 9 isclosest to channel 11 which, in turn, is closes to Detroit.

Returning now to FIG. 10, the conversion of a receive signal to atransmit signal will be described in connection with the examplementioned above in which the uplink site is in Detroit and the downlinksite is in Los Angeles. The uplink signal transmitted from Detroit wouldbe transmitted on channel 13 carried by receive signal R2. Thus, the R2receive signal is input to transmission line 122 and a portion of suchinput signal is diverted by the hybrid coupler 130 to the input line ofsubgroup 108 of filters 102. Subgroup 108 includes a bank of eightfilters for the odd channels, including channel 13. Thus, the incomingsignal is filtered through by filter 13 and is output on a line 164along with other signals from subgroups 108 and 116. The channel 13signal present on line 164, is combined by the hybrid coupler 158, withsignals emanating from subgroup 106 and 114, and forms the T4 signal onoutput line 150. The transmit signal T4 is then downlinked to LosAngeles.

It is to be understood that the above example is somewhat simplifiedinasmuch as the network control center would assign a more specificchannel than a 27 MHz wide band channel, since the 27 MHz wide channelmay actually comprise a multiplicity of smaller channels, for example,800 subchannels of 32 KHz bandwidth.

Referring now again to FIGS. 5, 8 and 9, in the event that an uplinksignal originates from one of the area of contention, 40, 42, 44 (FIG.5), such signal will not only be transmitted to its desired downlinkdestination, but a non-neglible signal will be transmitted to anothergeographic area. For example, assume that the uplink signal originatesfrom Chicago, Ill. which is in the area of contention 42 and that thesignal is destined for Los Angeles, Calif. The area of contention 42 isproduced by the overlap of the beams forming zones 34 and 36. Hence, theuplink signal can be transmitted as received signals R2 or R3. Thenetwork control center determines whether the uplink communication iscarried by receive signals R2 or R3. In the present example, sinceChicago is closer to zone 36, the uplink communication is carried onreceive signal R3.

As previously discussed, the downlink destination, Los Angeles, islocated in zone 37 and lies between channels 14 and 15. As shown in FIG.8, the intersection of R3 with column T4 yields the possible channelsover which the communication can be routed. Thus, the Chicago uplinksignal will be transmitted over one of channels 2, 6, 10 or 14. SinceLos Angeles is closest to channel 14, channel 14 is selected by thenetwork control center as the uplink channel. Note, however, that anundesired signal is also transmitted from zone 34 on channel 14. Todetermine where the undesired signal will be downlinked, the table ofFIG. 8 is consulted. The table of FIG. 8 reveals that uplink signalscarried on channel 14 in the R2 zone 34 are downlinked to the T1transmit zone 31. The desired signal is transmitted to Los Angeles andthe undesired signal (i.e. an extraneous signal) is transmitted to theEast Coast (i.e. zone 31). The network control center keeps track ofthese extraneous signals when making frequency assignments. The effectof these extraneous signals is to reduce slightly the capacity of thesystem.

Referring now again to FIG. 6, the beam-forming network 98 receives thetransmit signals T1-T4 and functions to couple all of the individualcommunication signals in these transmit signals together so that atransmit antenna beam for each signal is formed. In the examplediscussed above in which the assigned frequency spectrum is 500 MHz, atotal of approximately 50,000 overlapping antenna beams are formed bythe beam-forming network 98 when the system is fully loaded with narrowband signals. Each antenna beam is formed in a manner so that it can bepointed in a direction which optimizes the performance of the system.The incremental phase shift between adjacent elements determines thedirection of the antenna beam. Since this phase shift is determined bythe signal frequency, the system is referred to as frequency addressed.

Attention is now directed to FIGS. 11 and 12 which depict the details ofthe beam-forming network 98. The beam-forming network, generallyindicated by the numeral 98 in FIG. 11, is arranged in the general formof an arc and may be conveniently mounted on the communication shelf(not shown) of the satellite. The arc shape of the beam-forming network98 facilitates an arrangement which assures that the paths of thesignals passing therethrough are of correct length.

The beam-forming network 98 includes a first set of circumferentiallyextending transmission delay lines 168, 170, a second set oftransmission delay lines 172, 174 which are radially spaced from delaylines 168 and 170, and a plurality of radially extending waveguideassemblies 176. In the illustrated embodiment, forty waveguideassemblies 176 are provided, one for each of the elements 106 of thetransmit array 20 (FIG. 13). The waveguide assemblies 176 intersect eachof the delay lines 168-174 and are equally spaced in angle.

Each of the waveguide assemblies 176 defines a radial line summer andintersects each of the delay lines 168-174. As shown in FIG. 12, at thepoints of intersection, between the radial line summers 176 and thetransmission delay lines 168-174, a crossguide coupler 180 is provided.The crossguide coupler 180 connects the delay lines 168-174 with theradial line summers 176. The function of the crossguide couplers 180will be discussed later in more detail.

Four delay lines 168-174 are provided respectively for the four transmitzones T1-T4 (FIG. 9). Hence, transmit signal T1 is provided to the inputof delay line 170, T2 is provided to input of delay line 168, T3 isprovided to the input of delay line 174, and T4 is provided to the inputof delay line 173. As shown in FIG. 12, the distance between the radialline summers is indicated by the letter "I" and the width of each of theradial delay lines is designated by the letter "w". Although the radialline summers 176 are spaced at equal angular intervals along the delaylines 168-174, the distance between them varies from delay line to delayline due to the fact that the delay lines 168-174 are radially spacedfrom each other. Thus, the further from the center of the arc, which isformed by the radial line summers 176, the greater the distance betweenthe radial line summers 176, at the point where they intersect with thedelay lines 168-174. In other words, the spacing "I" between radial linesummers 176 for delay line 168 is less than the spacing between adjacentradial line summers 176 than for delay line 174. Typical values for thedimensions "I" and "w" are as follows:

    ______________________________________                                        Delay Line Signal      l. inches                                                                              w. inches                                     ______________________________________                                        168        T2          1.66     0.64                                          170        T1          1.72     0.66                                          172        T4          2.45     0.74                                          174        T3          2.55     0.76                                          ______________________________________                                    

The width of the delay lines 168-174, "w", and the distance "I" betweenadjacent radial line summers are chosen to provide the desired centerbeam squint and beam scan rate so that the beam pointing is correct foreach channel. This results in the desired start and stop points for eachof the transmit zones T1-T4.

Referring particularly to FIG. 12, the transmit signal T2 propagatesdown the delay line 168 for a precise distance, at which point itreaches the first radial line summer 176. A portion of the T2 signalpasses through the crossguide coupler 180, which may, for example, be a20 dB coupler, such that one percent of the transmitted power oftransmit signal T2 is diverted down the radial line summer 176. Thisdiverted energy then propagates down the waveguide 176 towards acorresponding solid state power amplifier 100 (FIGS. 6 and 11). Thisprocess is repeated for signal T1 which propagates down delay line 170.The portions of signals T1 and T2 which are diverted by the crossguidecouplers 180 (i.e. 0.01 T1 and 0.01 T2) are summed together in theradial line summer 176 and the combined signal 0.01 (T1 +T2) propagatesradially outwardly toward the next set of delay lines 172, 174. Thissame coupling process is repeated for signals T3 and T4 in delay line174 and 172 respectively. That is, 0.01 of signals T3 and T4 are coupledvia crossguide couplers 180 to the radial line summer 176. The resultingcombined signal 0.01 (T1+T2+T3+T4) propagates radially outwardly to anassociated solid state power amplifier 100 where it is amplified inpreparation for transmission.

After encountering the first radial line summer 176, the remaining 0.99of signals T1-T4 propagate to the second radial line summer where anadditional one percent of the signals is diverted to the summer 176.This process of diverting one percent of the signals T1-T4 is repeatedfor each of the radial line summers 176.

The signals, propagating through the radial line summers 176 towards theSSPAs 100, are a mixture of all four point-to-point transmit signalsT1-T4. However, each of the transmit signals T1-T4 may comprise 12,500subsignals. Consequently, the forty signals propagating through theradial line summers 176 may be a mixture of all 50,000 signals in thecase of the embodiment mentioned above where the assigned frequencyspectrum in 500 MHz wide. Therefore, each of the SSPAs 100 amplifies all50,000 signals which emanate from each of the plurality of wave guideassemblies 176.

Since each of the SSPAs 100 amplifies all 50,000 signals which aredestined for all regions of the country, it can be appreciated that allof the narrow, high gain downlink beams are formed from a common pool oftransmitters, i.e. all of the SSPAs 100. This arrangement may be thoughtof as effectively providing a nationwide pool of power since each of thedownlink beams covering the entire country is produced using all of theSSPAs 100. Consequently, it is possible to divert a portion of thisnationwide pool of power to accommodate specific, disadvantaged downlinkusers on an individual basis without materially reducing the signalpower of the other beams. For example, a downlink user may be"disadvantaged" by rain in the downlink destination which attenuates thesignal strength of the beam. Such a rain disadvantaged user may beindividually accommodated by increasing the signal strength of thecorresponding uplink beam. This is accomplished by diverting to thedisadvantaged downlink beam, a small portion of the power from the poolof nationwide transmitter power (i.e. a fraction of the power suppliedby all of the SSPAs 100). The power of an individual uplink beam isproportional to that of the corresponding downlink beam. Consequently,in order to increase the power of the downlink beam it is merelynecessary to increase the power of the uplink beam.

In practice, the previously mentioned network control center keeps trackof all of those regions of the country in which it is raining anddetermines which of the uplink users are placing communications todownlink destinations in rain affected areas. The network control centerthen instructs each of these uplink users, using packet switchedsignals, to increase its uplink power for those signals destined for arain affected area. The increase in power of the uplink user's signalsresults in greater collective amplification of these signals by theSSPAs 100, to produce corresponding downlink beams to the rain affectedareas, which have power levels increased sufficiently to compensate forrain attenuation. Typically, the number of signals destined for rainaffected areas is small relative to the total number of signals beinghandled by the total pool of SSPAs 100. Accordingly, other downlinkusers not in the rain affected zones do not suffer substantial signalloss since the small loss that may occur in their signals is spread outover the many thousand users.

The SSPAs 100 (FIGS. 8 and 11) may be mounted, for example, on the rimof the communication shelf (not shown) of the satellite. The signalsamplified by the SSPAs 100 are fed into the corresponding elements 106of the transmit array 20 (FIGS. 13 and 14).

As previously discussed, an incremental phase shift is achieved betweenthe signals that are coupled in the forty radial line summers 176.Hence, the beam-forming network 98 permits the antenna beams emanatingfrom the transmit array 30 (FIGS. 1, 2 and 13) to be steered byfrequency assignment. The incremental phase shift is related to the timedelay between the waveguides 176 as well as frequency. Attention is nowdirected to FIG. 17 which is a diagrammatic view of four of the fortytransmit array elements 106 (FIG. 13), showing the wavefront emanatingtherefrom, wherein "d" is equal to the spacing between transmit arrayelements 106. The resulting antenna beam has an angular tilt of θ, whereθ is defined as the beam scan angle. This means that θ is the angle fromnormal of the transmit beam center. The incremental phase shift producedby the delay line arrangement is ΔΦ. The relationship between ΔΦ and θis given by ##EQU1## where: λ=signal wavelength

θ=beam scan angle

d =spacing between array elements Hence, the east-west direction of theantenna beam is determined by the incremental phase shift which isdifferent for the four delay lines 168-174 of the beam-forming network98, resulting in the four transmit zones T1-T4 previously noted.

Reference is now made to FIGS. 18 and 19 wherein still another alternateform of the invention illustrated in FIGS. 1-19 is depicted. In thealternate embodiment, the satellite 200 employs a direct radiating arrayantenna 202 for both the reception and transmission and is designed forL band and Ku band service. In FIG. 18, the satellite 200 is illustratedin its deployed position. The satellite 200 is body-stabilized, and hassolar panels 206 and antenna arrays 202, 204 arranged so that they maybe stowed, as illustrated in FIG. 19, in a compact arrangement duringlaunch. Specifically, the outer portions of the L band array 202 arehinged so that they may be swung inwardly to a stowed position forlaunch. Similarly, the solar panels 206 which occupy the north and southfaces of the body-stabilized bus are hinged so that they may be stowedcompactly for launch and the Ku band array 204 is positioned so that itfacilitates the stowing of the satellite during launch. Referring nowalso to FIG. 20, the L band directly radiating array antenna 202 is aclose-packed, two-dimensional array of circularly-polarized cuppeddipoles 205. In the illustrated embodiment, the array 202 is 2.4 m by8.0 m, and has thirty-two columns, where each column of dipoles indriven in pairs 207 so that only sixteen drive points are needed. Thearray 202 forms both transmit and receive beams.

Referring to FIG. 21 and 22 concurrently, a typical geographic area(greater North America) serviced by the satellite 202 is illustrated.The satellite system is designed to reuse the L band frequency spectrumthree times, the frequency band being reused in each zone 206, 208, 210.In each zone 206, 208, 210, the low end of the frequency band isdirected to the west edge of the zone, and the high end is directed tothe east edge of the zone. As illustrated in FIG. 22, where the zones209, 211 meet, the high-frequency end of one zone coincides with the lowfrequency end of the other zone. By using a tapered illumination of thearray, suitably low side lobe levels are generated so that theinterference caused by the simultaneous use of each frequency channelthree times is reduced to acceptable levels.

In order to achieve reuse of the frequency spectrum three times, athree-zone beam-forming network 212, as shown in FIG. 23, is employed.The beam-forming network 212 functions similar to the previouslydiscussed beam-forming network illustrated in FIG. 11. The beam-formingnetwork 212 also includes transmission delay lines 214 wherein there isone delay line associated with each zone 206, 208, 210, and there arethree transmission lines 216 for each pair of the sixteen drive pointsin the array antenna 202. The outputs of the three zonal signals areadded by hybrid couplers 218 to form single outputs corresponding to theantenna drive points.

Reference is now made to FIG. 24, wherein a block diagram of thesatellite 200 is shown which is particularly suited for use with mobileground stations. With mobile service, the mobile users and thestationary parties are connected by base stations.

The satellite 200 and the base stations for the fixed terminal servicecommunicates over assigned frequencies in the Ku-band, whereas thesatellite 200 and the mobile service users communicate over assignedfrequencies in the L band on both the uplink and downlink. Ku-bandsignals from base stations are received by the Ku array antenna 204 andreceivers 257. The signal frequency is converted from Ku-band to L-bandby down converters 255. The converted L-band signals from the multiplezones, are combined in a three zone beam-forming network 212, amplifiedby dual power amplifiers 251, divided by a diplexer 259, and transmittedto mobile service users via the L-band array 202. This is referred to asthe "forward communication link". The "return communication link", fromthe mobile users is transmitted to the satellite 200 over L-band andreceived by the L-band array 202. The L-band signals from differentreceive zones, are amplified by amplifier 263, combined by a beamforming network 212, and upconverted 253 to the Ku-band frequency. Thesignals are transmitted 261 via the Ku-band array 204 to the basestations in various zones.

Reference is now made to FIG. 25, wherein the power distribution 252 forthe array 202 is shown. In order to avoid the design and productioncosts associated with systems employing several multiple level poweramplifiers, the unequal power levels, required by the array elements205, are generated by hybrid-coupled dual amplifiers 251. In thisarrangement, there are eight hybrid-coupled dual amplifiers wherein allof the amplifiers operate at the same power level despite the unequalpower levels at their inputs and outputs. By the use of hybrid-coupleddual amplifiers 251, the intermodulation products are dispersedgeographically, thus reducing their level at the user's locations. Inaddition, by using power amplifiers 251, in which each amplifiercontributes to the amplification of all of the signals, maximumflexibility and power assignment is achieved.

Referring to FIG. 26, another alternate embodiment of the invention isillustrated wherein the satellite system provides for a private Ku-bandnetwork system 250 utilizing four direct-radiating arrays: two receivearrays 235, 236 and two transmit arrays 237, 238. There are two arraysfor both the transmit and receive because both the broadcast andpoint-to-point services are accommodated on a single polarization. Onereceive array 235 is dedicated for horizontal polarization while thesecond receive array 236 is dedicated to vertical polarization.Similarly, one transmit array 237 is dedicated for horizontalpolarization and the second transmit 238 is dedicated for the verticalpolarization. Both services are made available on one polarization byallocating one half of the frequency spectrum to broadcast service andone half of the spectrum to point-to-point service. By providing bothservices on one polarization, the aperture of the ground terminals maybe small, thus reducing the cost of the ground stations. Two parabolicreflectors 244 are used to provide shaped beam coverage over thegeographic region serviced by the satellite. The Ku-band domesticsystem, like the L band mobile system, is able to overcome rainattenuation on the downlink with only slight penalty to the signals inrain-free areas. Since only a small portion of the country experiencesrain at one time, large power increases to those disadvantaged areascauses only a slight decrease in power available elsewhere. As with theL band mobile system, the solar panels 248 are extendable and are stowedduring launch.

The illustrated Ku-band satellite system permits frequency reuse in theprivate network system by utilizing very small aperture earth terminalson user premises. The private network may be either a "star" network,which connects the earth terminals to a central data base and providesbroadcast video capacity, or a "mesh" network, which connects the earthterminals by circuit-switched interconnections to provide services suchas teleconferencing, high-speed graphics, and voice communication.

Attention is now directed to FIG. 27 which illustrates contiguousreceive zones 220, 222, 224, 226 for a typical domestic Ku-band privatenetwork satellite system. In this system, pairs of users are directlyconnected without the use of intermediary base stations, thus precludingthe use of a frequency addressable beam on both the uplink and downlink.Consequently, a set of frequency independent beams is used on theuplink, so that frequency may be used to address the downlinkdestination.

There are four receive zones 220, 222, 224, 226 servicing the areabecause the frequency spectrum is reused on the uplink four times, theentire frequency spectrum being used in each of the zones 220, 222, 224,226. In order for each uplink zone to be contiguous and non-interfering,each zone is further subdivided into two half zones, illustrated in FIG.27, 220a and 220b, 222a and 222b, 224a and 224b, and 226a and 226b. Thereceive beam patterns corresponding to zones 220a, 220b, 222a, 222b,224a, 224b, 226a, and 226b are illustrated in FIG. 28. FIG. 28 shows thecenter frequency for each receive beam. The beam patterns 221, 223, 225,227, 229, 231, 233 may be generated, for example by an array 80wavelengths wide driven at 16 drive points through a Butler matrixhaving 16 antenna terminals and providing 8 beam outputs. The beampatterns 221, 223, 225, 227, 229, 231, 233, 235 have sufficient spatialisolation to permit frequency reuse among the beams separated by atleast one beam but not among adjacent beams. The satellite uses fourtwo-position time switches so that the "a" and "b" beams are usedalternately; this is effectively a type of time division arrangement.The switches, operating at the low speed of, for example, 50 Hz, providefour receive zone outputs to the four receivers that amplify and convertthe signals from 14 to 12 GHz. Using a 50 Hz switch and creating a 20msec period means that during one time period while the "a" frequenciesare simultaneously used in the four zones, the "b" set of frequenciesare not "on" and thus are not used. Hence, the "a" and "b" frequencieswill not interfere with one another. Since the duty cycle is one half,the power requirement is doubled at each terminal.

Referring also to FIG. 29 and 30, the transmit zones and the transmitbeam patterns are respectively shown. The transmit zones 228, 230, 232,234 illustrate in FIG. 29 are essentially the same as the receive zones220, 222, 224, 226 illustrated in FIG. 27. The transmit zone 228corresponds to the receive zone 220, transmit zone 230 corresponds toreceive zone 222, transmit zone 232 corresponds to the receive zone 224,and transmit zone 234 corresponds to receive zone 226. As with theuplink, the downlink spectrum is also reused four times, the entirespectrum being reused in each of the transmit zones 228, 230, 232, 234.As shown in FIG. 30, each of the transmit beams 237, 239, 241, 243respectively correspond to the transmit zones 228, 230, 232, 234.

Referring to FIG. 31, a filter interconnect matrix 252 is shown whichincludes an assembly of thirty-six filters that provides the connectionsbetween the receive zones 220a, 220b, 222a, 222b, 224a, 224b, 226a, 226b(R1A, R1B, R2A, R2B, R3A, R3B, R4A, R4B) and transmit zones 228, 230,232, 234 (T1, T2, T3, T4). The filter interconnection matrix 252 issimilar to the previously discussed filter matrix 90. The rows of thematrix are driven by the receiver outputs, and the preselected groups ofoutput from nine filters are directed to the four transmit zones T1, T2,T3, T4.

Referring to FIG. 32, a block diagram of the alternate embodiment of thepresent invention is illustrated. Signals received by the receive arrayantenna 236 enter a beam forming network 265, similar to the previouslydiscussed BFN 98 and 212 to combine the received signals. The combinedsignals are routed to receivers 269 by time division switches 267 wherethe signals are preselectively filtered by a filter interconnectionmatrix 252 which performs in substantially the same manner as thepreviously discussed FIM 90. The filtered signals to be transmitted arealso combined in a beam forming network 265 after which the signals areamplified at 273 and transmitted by the directly radiating transmitarray antenna 238.

Having thus described the invention, it is recognized that those skilledin the art may make various modifications or additions to the preferredembodiment chosen to illustrate the invention without departing from thespirit and scope of the present contribution to the art. Accordingly, itis to be understood that the protection sought and to be afforded herebyshould be deemed to extend to the subject matter claimed and allequivalents thereof fairly within the scope of the invention.

What is claimed is:
 1. A method of communicatively interconnecting anyof a plurality of terminals within an area on the earth using anearth-orbiting communications satellite, comprising the steps of:(A)forming a first plurality of groups of radio frequency beamsrespectively between each of a plurality of essentially contiguous zonescovering said area on the earth and said satellite, the beams in saidgroups thereof carrying communications signals over the same firstpreselected range of frequencies, such that said first preselected rangeof frequencies is reused by the beams for all of said zones; (B)separating each group of said radio frequency beams into first andsecond sets thereof respectively carrying communications signals overfirst and second sets of frequencies within said first preselected rangeof frequencies, and spatially distributing said first preselected rangeof frequencies over each of the zones as a function of frequency suchthat the frequencies of one set of said beams in one zone is spatiallyseparated from the frequencies of one set of said beams in a zonesubstantially contiguous to said one zone; (C) alternately transmittingthe first and second sets of said beams between said zones and saidsatellite by simultaneously transmitting the beams in the first setthereof for all the zones and then simultaneously transmitting the beamsin the second set thereof for all the zones.
 2. The method of claim 1,wherein step (C) is performed by transmitting the first and second setsof beams from each of said zones to said satellite.
 3. The method ofclaim 2, including the steps of:(D) receiving said first and second setsof beams at said satellite; and (E) transmitting a second plurality ofgroups of radio frequency beams respectively from said satellite to saidzones, the beams in each group of said second plurality thereof carryingcommunications signals over the same second preselected range offrequencies, whereby said second preselected range of frequencies isreused by all of the groups of beams in said second plurality thereof.4. The method of claim 3, wherein step (E) is performed by spatiallydistributing the beam in each of said second plurality of groups thereofacross the corresponding zone as a function of said second preselectedrange of frequencies.
 5. A method of communicatively interconnecting aplurality of terminals within an area on the earth using an earthorbiting communications satellite, comprising the steps of:(A) dividingsaid area into a plurality of essentially contiguous zones; (B) dividingeach zone into at least first and second contiguous sections; (C)alternately transmitting at least first and second radio frequency beamsrespectively between said first and second sections for each of saidzones and said satellite, said first and second beams respectivelycarrying communications signals over first and second different sets offrequencies defining a first preselected range of frequencies, whereinsaid first preselected range of frequencies is reused by the first andsecond beams for all of said zones, and (D) spatially distributing thefrequencies in the beams respectively transmitted to the first andsecond sections of each of the zones as a function of frequency.
 6. Themethod of claim 5, including the step of forming said first and secondbeams for each of said zones such that said first and second beam foreach of said zones collectively define a rectangularly shaped area ofcommunications service on the earth which corresponds to the associatedzone.
 7. The method of claim 5, wherein the sets of frequencies of saidcontiguous two sections are different from each other, whereby to avoidcommunications interference between said contiguous two sections.
 8. Themethod of claim 5, including the steps of:(D) receiving said first andsecond beams at said satellite; and (E) transmitting to each of saidzones a plurality of downlink radio beams, each plurality of saiddownlink beams carrying communications signals over the same, secondpreselected range of frequencies, whereby said second preselected rangeof frequencies is reused by the downlink beams for all of said zones. 9.The method of claim 8, wherein step (E) includes the step of spatiallydisbursing the downlink beams in each plurality thereof as a function offrequency.
 10. A satellite communications system for communicativelyinterconnecting a plurality of earth terminals covering an area on theearth, comprising:a satellite disposed in orbit above the earth; firstmeans for forming a plurality of uplink radio frequency beams betweensaid satellite and each of a plurality of respectively associatedcontiguous zones covering said area, said zones being divided into atleast two essentially contiguous sections, each plurality of said uplinkbeams carrying communications signals destined to be received bydownlink terminal sites in said zones, the uplink beams for each zonebeing arranged in first and second sets of said beams respectivelyoriginating from said two sections in the zone and carryingcommunication signals over first and second sets of frequencies, saidfirst and second set of frequencies defining a first range offrequencies and being respectively spatially distributed over said twosections as a function of frequency, the uplink beams for all of saidzones using said first range of frequencies; second means at saidsatellite for alternately receiving in time said first and second setsof beams to prevent interference between signals originating fromadjacent ones of said sections; and third means at said satellitecoupled with said second means for forming a plurality of groups ofdownlink radio frequency beams between said satellite and said zones,each of said groups of downlink beams carrying communication signalsdestined to be received at downlink terminal sites in one of said zones.11. The system of claim 10, wherein each of said groups of downlinkbeams covers one of said zones with each group carrying a plurality ofsignals over a second range of frequencies such that the same range offrequencies are used by all the groups of downlink beams.
 12. The systemof claim 10, wherein said second means includes a receiving antennaarray, a receiver and a two position switch for successively switchingsaid receiver to alternately receive said first and second sets offrequencies respectively.
 13. The system of claim 12, wherein said thirdmeans includes a transmitting antenna array for transmitting saiddownlink beams, said receiving antenna array and said transmittingantenna array each being defined by a two dimensional array of directradiating elements.